There are many situations in both human and veterinary medicine where it is desirable to rapidly reverse hyperthermia. There are also many clinical situations where it is essential to be able to rapidly reduce dangerously elevated body temperatures to near normal, as in the case of hyperthermia from heat stroke, drug or surgical anesthetic reaction, and febrile illness secondary to stroke, infection, or other illness. In fact, it has been demonstrated in several studies that patient mortality is directly dependent on the length of time a patient has a high body temperature, and inversely dependent on the rapidity with which core temperature is normalized. Further, it has been recently demonstrated that for patients suffering from post-resuscitation encephalopathy after recovery from a period of cardiac arrest, inducing hypothermia as an adjunct to other therapies after heartbeat is restored significantly increases survival rates and rates of discharge from hospital to functional living.
This application refers to and incorporates herein by reference U.S. Pat. No. 6,694,977, titled Mixed-Mode Liquid Ventilation Gas and Heat Exchange (hereinafter “MMLV patent”), in which a method of Mixed-Mode Liquid Ventilation (“MMLV”) and a device (“Prior Device”) for the administration of MMLV is disclosed for rapidly inducing or reversing hypothermia. The method comprises the continuous delivery and removal of perfluorocarbon to and from the lungs, while also providing for the delivery of gas breaths by means of a mechanical ventilator or other device at a rate that is independent of the delivery and removal of perfluorocarbon from the lungs. The inventors of the present apparatus have discovered, however, that when the purpose of the MMLV is to only induce hyperthermia in order to decrease core mammalian temperature, continuous delivery and removal of perfluorocarbon to and from the lungs need not be accompanied by the delivery of gas breaths that are independent of perfluorocarbon delivery and removal rates. Rather, the delivery of gas breaths can be synchronized with perfluorocarbon infusion or can be delivered at a rate independent of perfluorocarbon infusion. This discovery has, in part, lead to the development of a new apparatus and method for the administration of heat exchange in the lungs of a mammal that constitutes a substantial improvement over the prior heat exchange device and method disclosed in the MMLV patent.
Although the Prior Device has performed its functions well in the laboratory setting, its continual use over the years has revealed many undesirable features. One such limitation is that the Prior Device is cumbersome and not easily transported form one location to another due to the fact that the device consists of a perfluorocarbon tank containing perfluorocarbon, a separate vacuum reservoir tank to serve as a collection suction reservoir, a large peristaltic pump to infuse cold perfluorocarbon liquid, a vacuum pump to maintain the suction reservoir, a separate ice water tank containing ice water and a heat exchanger. Finally, the Prior Device contained a separate silicone membrane oxygenator unit, to add oxygen to the perfluorocarbon and remove carbon dioxide from it. In addition, due to the separation of the perfluorocarbon and the ice water tanks, long tubing must be utilized to transfer the perfluorocarbon from the perfluorocarbon tank to the heat exchanger where the perfluorocarbon is cooled before it is infused into the lungs of a patient. This results in an increase in the temperature of the perfluorocarbon during transit. Another difficulty that has been encountered with a later version of the Prior Device is that it utilizes a weighing system to meter the volume of perfluorocarbon contained within the perfluorocarbon tank and the weight is monitored using the LabView® program operating on a computer. This feature has proven to be overly complicated, failure-prone, heavy, and required a significant amount of electrical power. In yet another version of the Prior Device, which used no vacuum pumps but only peristaltic pumps, the apparatus used stepper motors to operate an infusion pinch valve to control the flow of perfluorocarbon to the patient, a suction pinch valve to control the flow of perfluorocarbon from the patient, and a recycling pinch valve to recycle the perfluorocarbon from the heat exchanger to the perfluorocarbon tank and back to the exchanger. Due to the nature of stepper motors they require a dedicated electronic circuit in order to operate the motors, which again added to the size, weight, complexity, and power consumption.
Another limitation of the Prior Device is that it was designed such that the infusion/suction tube was concentric with the endotracheal tube, and the end of the infusion tube was perforated in order to minimize potential damage to the lung tissue. These two features resulted in a substantial limitation on the volumes of perfluorocarbon that could be delivered to and removed from the lungs, and as result limited the rate of heat exchange in the lungs of canines to about 1.5° C. within 5 minutes. In addition the Prior Device used an occlusive pump for infusion and a large centrifugal pump to circulate ice water through a heat exchanger. Both pumps required 110v AC electrical current connections, were heavy, and were relatively inefficient. They were, therefore, unsuitable for applications requiring portability of the equipment. Previous versions of the apparatus also were used in conjunction with a mechanical ventilator, which was heavy, cumbersome, non-portable, and could not be coordinated with liquid infusion and removal.
Lastly, the Prior Device incorporated a gas exchanger to add oxygen to or remove carbon dioxide from the perfluorocarbon liquid, as would be appropriate for total liquid ventilation. These gas exchangers, under conditions of 100% oxygen gas ventilation, were eventually replaced by a system of only absorbing carbon dioxide, relying on a pure oxygen inflow. Ultimately, however, it became clear that very small amounts of perfluorocarbon, on the order of 50% of the lung Functional Residual Capacity (FRC, ordinarily about 15 mL/kg), could be used for liquid infusion. This discovery suggested that the gas exchanger and CO2 absorption system might not be needed, and ultimately lead to the use of a much simpler and more effective gas ventilation system described in this patent application.
Overall, the foregoing limitations of the Prior Device resulted in a device that was not sufficiently reliable and portable to be used by paramedics or other emergency personnel away from a medical setting with access to highly skilled, licensed physicians and the Prior Device exhibited heat exchange cooling rates that were potentially too slow to be successfully used in an emergency setting.